define(['zepto', 'touch'], function($) {
	var bbTop = function() {
		var id = setInterval(function() {
			var scrollTop = $('body').scrollTop();
			var speed = scrollTop / -4;
			scrollBy(0, Math.floor(speed));
			if(scrollTop <= 0) {
				clearInterval(id);
			}
		}, 30);
	}
	$('.back-top').tap(function() {
		bbTop();
	});
	$('.backtop').tap(function(){
		bbTop();
	})
	$(window).scroll(function() {
		var reTop = $('.back-top');
		var scrollTop = $('body').scrollTop();
		var screenHeight = $(window).height();
		if(scrollTop >= (screenHeight)) {
			reTop.show();
		} else {
			reTop.hide();
		}
	});
});